1. Field of the Invention
The present invention relates generally to a solid waste purification method and more particularly, to a method that can effectively separate and purify potassium nitrate from a solid waste, which contains potassium nitrate and sodium nitrate and is produced by a chemical glass-strengthening process. The present invention also relates to an apparatus for implementing the purification method.
2. Description of the Related Art
Generally, there are two types of glass-strengthening processes, including physical strengthening treatment and chemical strengthening treatment. In the chemical strengthening treatment, the ion exchange reaction will occur when a glass of sodium silicate is immersed in molten potassium nitrate to replace sodium ions contained in the glass with potassium ions contained in the molten potassium nitrate, that is, small sodium ions in the glass are substituted by larger potassium ions, such that the surface strength of the glass is improved.
After the ion exchange reaction is proceeded for a period of time, the potassium nitrate contained in the molten salt thereof will be gradually displaced into sodium nitrate. Although the molten potassium nitrate in industrial grade inherently contains a minor amount of sodium ion, when the concentration of sodium nitrate contained in the reacted molten potassium nitrate increases to a certain extent, for example, the concentration of sodium nitrate may increase several ten times due to the ion exchange reaction, the ion exchange reaction may not be continuously conducted, resulting in that the surface strength of the glass cannot be effectively improved to a desired grade.
Therefore, although the reacted molten salt still contains a certain amount of potassium nitrate, it cannot however be applied in chemical strengthening treatment again. In order to reduce the cost, a part of the reacted molten salt is diluted with pure molten potassium nitrate and then the diluted molten salt is used in chemical strengthening treatment; however, the remaining part of the reacted molten salt may be useless and discarded. This is wasteful and environmentally unfriendly. In addition, because a large amount of pure molten potassium nitrate is used to dilute the reacted molten salt, the manufacturing cost is undesirably increased.